Ischemic stroke occurs when a blockage in an artery leading to the brain causes a lack of supply of oxygen and nutrients to the brain tissue. The brain relies on its arteries to supply oxygenated blood from the heart and lungs. The blood returning from the brain carries carbon dioxide and cellular waste. Blockages that interfere with this supply eventually cause the brain tissue to stop functioning. If the disruption in supply occurs for a sufficient amount of time, the continued lack of nutrients and oxygen causes irreversible cell death (infarction). Accordingly, immediate medical treatment of an ischemic stroke is critical for the recovery of a patient.
The infarction may not develop or may be greatly limited, given a rapid clearing of the blockage to reestablish the flow of blood. However, if left untreated, ischemic stroke may lead to the permanent loss of brain tissue, and can be marked by full or partial paralysis, loss of motor control, memory loss, or death.
Several different diseases may lead to an ischemic stroke. Typically, deposition of cholesterol (artheroselerosis), formation of blood clots, or other objects in the vessels may disrupt blood flow and lead to ischemic stroke. Furthermore, the substances that cause the blockages may break free from larger vessels outside the brain and become lodged within narrower arteries closer to the brain embolism).
Ischemic stroke may be divided into thrombotic strokes and embolic strokes. A thrombotic stroke occurs when the building and rupturing of atheromatous plaque within the brain blocks cerebral arteries. Clinically referred to as cerebral thrombosis or cerebral infarction, this condition represents approximately 10% of all strokes. An embolic stroke occurs when a clot or emboli forms somewhere other than in the brain, such as in the cervical carotid artery or in the heart, and travels in the bloodstream until the clot becomes lodged and can not travel any further. When such a condition occurs in the arteries supplying the brain, the condition results in almost immediate physical and neurological effects.
While these are the most common causes of ischemic stroke, there are many other possible causes. Examples include use of drugs, trauma to the blood vessels of the neck, or blood clotting disorders.
Apart from surgical techniques, medical practitioners could address such blockages with the use of Tissue Plasminogen Activator (t-PA). However, t-PA must be used within the first three hours of the onset of stroke symptoms and may take hours or even days to successfully restore flow. In addition, t-PA carries an increased risk of intracerebral hemorrhage. It is currently believed that the use of t-PA results in a 30% success rate as well as a 6% major complication rate. In view of these limitations, the majority of stroke patients in the U.S. do not receive t-PA treatment.
In addition, there are a number of surgical techniques used to remove blockages. For example, an embolectomy, involves incising a blood vessel and introducing a balloon tipped device (such as the Fogarty catheter) to the location of the occlusion. The balloon is then inflated at a point beyond the clot and used to translate the obstructing material back to the point of incision. The obstructing material is then removed by the surgeon. Concentric Medical, Inc. of Mountain View, Calif. supplies devices for an interventional approach to the removal of obstructions. Concentric supplies a Merci® Retriever system as a device based approach for the removal of clots. This system engages and ensnares a clot. Once captured, a balloon catheter inflates to temporarily halt forward blood flow while the clot is withdrawn. The clot, is then pulled into the catheter and out of the body.
Typically, the existing means to remove obstructions do not address the frictional forces that act on the obstruction during removal of the obstruction. For example, some conventional devices engage the clot from the distal (or downstream) side. As the device is pulled proximally (or upstream), the device attempts to either engulf or ensnare the clot. However, due to the consistency of the clot and because the clot is typically well lodged, within the vessel, the act of pulling, the clot in a proximal direction cause the clot to also compress in an axial direction. This axial compression (when viewed along the axis of the vessel) causes a contemporaneous radial expansion of the clot (when viewed relative to the vessel). As a result, the increase in diameter of the clot causes an increase in the frictional forces applied against the arterial wall. Thus, by not addressing the frictional forces acting on the obstruction, the process of removing the clot may actually increase the static force that would otherwise be required to remove or translate the clot within the vessel. Unfortunately, increasing the amount of force applied upon one side of the clot also increases the probability of complications during the procedure (e.g., fragmenting the clot, failing to remove the clot, failure to fully engulf/ensnare the clot, and/or device failure) and can cause potential damage to the surrounding vessel.
While there are other drugs and suppliers of devices for removal of blockages, there remains a need for methods and devices that improve the success rate and/or reduce the complication rate in restoring flow and thereby limit the damage from an ischemic stroke.